1. Field of the Invention
This invention relates to a silver halide photographic emulsion and more particularly to a silver halide photographic emulsion spectrally sensitized with a combination of sensitizing dyes.
2. Description of the Prior Art
Various methods have been known for the spectral sensitization of silver halide photographic emulsions. Various sensitizing dyes have been employed for their spectral sensitization in the red, green or blue regions of the spectrum. Among the sensitizing dyes, merocyanine dyes have often been employed for spectral sensitization in the green and blue regions of the spectrum.
In photographic light-sensitive materials for graphic art use, particularly the so-called "lith type" light-sensitive materials, which are used for the production of halftone dots from originals with continuous tone employing a contact screen, merocyanine dyes have been employed for their spectral sensitization in the blue and red regions of the spectrum (for example, see U.S. Pat. Nos. 3,625,698, 3,480,439, 3,567,458, 2,519,001, 3,698,910, 3,703,377, 2,493,748, 2,493,747 and 2,497,876, Japanese Patent Publications 18,106/71, 18,105/71 and 549/71, U.S. Pat. Nos. 3,765,900 and 3,765,901, and Offenlegungschrifts 2,135,412, 2,101,071 and 2,057,034).
For the panchromatic sensitization of light-sensitive materials of this type, combinations of red-sensitive cyanine dyes and dimethine merocyanine dyes have mostly been employed (for example, see U.S. Pat. Nos. 3,808,009 and Offenlegungschrift 2,101,071). This is because merocyanine dyes can render silver chloride emulsions highly sensitive and can increase gradation (so-called contrast), which is advantageous for the production of lith type light-sensitive materials.
Spectral sensitization using merocyanine dyes is, however, by no means satisfactory. That is, it is very difficult to increase blue sensitivity using merocyanine dyes as mentioned above, in spite of intensive investigation. Green sensitivity is often lowered when hitherto known combinations of dimethine merocyanine dyes and simple merocyanine dyes (e.g., those described in Offenlegungschrift 2,101,071) are employed in order to improve blue sensitivity. Red sensitivity is often lowered when merocyanine dyes are employed in combination with red-sensitive thiacarbocyanine or selenacarbocyanine dyes. Namely, though merocyanine dyes are excellent sensitizing dyes for spectral sensitization in the blue or green regions of the spectrum, few combinations of merocyanine dyes and other sensitizing dyes with different structures having a remarkable synergistic effect or supersensitizing relation are known, and such combinations have only a weak effect.
Spectral sensitization using merocyanine dyes has hitherto also been insufficient with respect to other properties such as color stain. With the recent expansion of processing systems using automatic processors, fixing and washing have been carried out at minimum times, and hence color stain is liable to remain after development, which requires spectral sensitization techniques using dyes which leave minimal color stains and techniques which reduce color stain. In addition, spectral sensitization using dyes has also been insufficient with respect to the stability of the light-sensitive materials after production, that is, with respect to the sensitization, desensitization, increase in fog and, in the case of color photographic emulsions employing color couplers, reduction in green sensitivity with the passage of time.